An interval theory of probability is presented for use as a measure of evidential support in knowledge-based systems. An interval number is used to capture, in a relatively simple manner, features of fuzziness and incompleteness. The vertex method is used for the interval analysis. A new parameter (also an interval number), p, called the degree of' dependence is introduced. The relationship of this interval probability with the theories of Dempster-Shafer, fuzzy sets, and Baldwin's support logic are discussed. The advantage of the theory is that it is based on a development of the axioms of probability, but allows that evidential support for a conjecture be separated from evidential support for the negation of the conjecture.